Searching for Exoplanet Oceans More Challenging Than First Thought

As astronomers continue to discover more exoplanets, the focus has slowly shifted from what sizes such planets are, to what they’re made of. First attempts have been made at determining atmospheric composition but one of the most desirable finds wouldn’t be the gasses in the atmosphere, but the detection of liquid water which is a key ingredient for the formation of life as we know it. While this is a monumental challenge, various methods have been proposed, but a new study suggests that these methods may be overly optimistic.

One of the most promising methods was proposed in 2008 and considered the reflective properties of water oceans. In particular when the angle between a light source (a parent star) and an observer is small, the light is not reflected well and ends up being scattered into the ocean. However, if the angle is large, the light is reflected. This effect can be easily seen during sunset over the ocean when the angle is nearly 180° and the ocean waves are tipped with bright reflections and is known as specular reflection. This effect is illustrated in orbit around our own planet above and such effects were used on Saturn’s moon Titan to reveal the presence of lakes.

Translating this to exoplanets, this would imply that planets with oceans should reflect more light during their crescent phases than their gibbous phase. Thus, they proposed, we might detect oceans on extrasolar planets by the “glint” on their oceans. Even better, light reflecting off a smoother surface like water tends to be more polarized than it might be otherwise.

The first criticisms of this hypothesis came in 2010 when other astronomers pointed out that similar effects may be produced on planets with a thick cloud layer could mimic this glinting effect. Thus, the method would likely be invalid unless astronomers were able to accurately model the atmosphere to take its contribution into consideration.

The new paper brings additional challenges by further considering the way material would likely be distributed. Specifically, it is quite likely that planets in the habitable zones without oceans may have polar ice caps (like Mars) which are more reflective all around. Since the polar regions make up a larger percentage of the illuminated body in the crescent phase than during the gibbous, this would naturally lead to a relative diminishing in overall reflectivity and could give false positives for a glint.

This would be especially true for planets that are more oblique (are “tilted”). In this case, the poles receive more sunlight which makes the reflections from any ice caps even more pronounced and mask the effect further. The authors of the new study conclude that this as well as the other difficulties “severely limits the utility of specular reflection for detecting oceans on exoplanets.”

By Jon Voisey
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Jon is a science educator currently living in Missouri. He is a high school teacher and does outreach with the St. Louis Astronomical society as well as presenting talks on science and related topics at regional conventions. He graduated from the University of Kansas with his BS in Astronomy in 2008 and has maintained the Angry Astronomer blog since 2006.
For more of his work, you can find his website here.

When you take a photo, its build up out of many square colored points or pixels. A planet that far away and as small as earth will cover * less* than 1 pixel.
If you see the pixel slowly dim, than light up before it disappears … that’s what this article is talking about. A planet moving from full phase to crescent phase etc.

Brightness is only one kind information though. A pixel will most often contain information on 3 wavelength channels. And if someone finds a way to increase the pixel information to multi-range spectrum than astronomers will be happy. But the information density will be overloading any system though.

The link I give above is a treasure trove on what putative exoplanet characterization observations we can do. Spectral absorption and reflectance polarization has been mentioned.

To add to that, IIRC continental wide large regular structures (i.e. woods), can theoretically be observed by detecting wavefront information. This is a new and upcoming method in radio observations and signal coding, since the topology of the wavefront can encode a lot of information.

None of those methods are dependent on resolution. The problem is signal attenuation, exacerbated by having to filter out the star emission. Only the closest exoplanets will be characterized thusly, and well into the future.

But for that matter, imaging admits mapping planet surfaces due to well characterized telescope spreading functions and body rotation:

“Spitzer measured the infrared light coming from the planet as it circled around its star, revealing its different faces. These infrared measurements, comprising about a quarter of a million data points, were then assembled into pole-to-pole strips, and, ultimately, used to map the temperature of the entire surface of the cloudy, giant planet.”